Fiber-reinforced composite materials, which are excellent in strength, rigidity, dimensional stability, and other properties despite their light weight, have been widely used in general industrial fields, such as office machine applications, computer-related applications (e.g., IC trays, housings of notebook computers), and automobile applications, and increasingly demanded year by year. As reinforcing fibers in these materials, metal fibers such as aluminum fibers and stainless steel fibers, organic fibers such as aramid fibers and PBO fibers, inorganic fibers such as silicon carbide fibers, carbon fibers, and other fibers have been used. In terms of the balance of specific strength, specific rigidity, and lightness, carbon fibers are suitable, and in particular, polyacrylonitrile-based carbon fibers have been advantageously used.
As sizing agents for application to these reinforcing fibers, resins such as phenolic resins, melamine resins, bismaleimide resins, unsaturated polyester resins, and epoxy resin, have been advantageously used. In general, molded articles of fiber-reinforced composite materials made of reinforcing fibers and a matrix resin require high interfacial adhesion between the reinforcing fibers and the matrix resin in order to achieve high tensile strength and compression strength in the fiber direction. Thus, as sizing agents for application to reinforcing fibers, epoxy resins have been particularly advantageously used in order to improve the interfacial adhesion and provide a fiber-reinforced composite material having high mechanical strength. For example, there are proposed methods including applying a sizing agent of bisphenol A diglycidyl ether to carbon fibers (Patent Documents 1 and 2). In addition, there are proposed methods including applying a sizing agent of an epoxy adduct of polyalkylene glycol to carbon fibers (Patent Documents 3, 4, and 5).
Fiber-reinforced composite materials are ununiform materials and thus have great differences in physical properties in the direction of reinforcing fiber alignment and other directions. For example, shock resistance, indicated by resistance to falling weight impact, is known to be determined by interlaminar peel strength, i.e., GIc (opening mode) and GIIc (in-plane shearing mode) interlaminar fracture toughnesses. In particular, carbon fiber-reinforced composite materials including a thermosetting matrix resin are brittle to stresses from directions other than the direction of carbon fiber (reinforcing fiber) alignment due to the low toughness of the matrix resin.
To solve this problem, studies have been made on sizing agents capable of improving the strength against stresses from directions other than the direction of fiber alignment as well as the tensile strength and compression strength in the fiber direction. For example, there is proposed a method of producing a carbon fiber-reinforced composite material provided with high interlaminar fracture toughness by using a sizing agent necessarily containing a flexible epoxy resin and an epoxy resin incompatible with the flexible epoxy resin (Patent Document 6).